The invention is generally related to image processing systems and, more specifically, to a method and apparatus for low light imaging.
Night vision systems have become increasingly reliant on infrared imaging and detection in recent years. Present silicon imagers often require auxiliary cooling to achieve useful night vision imaging. This is because of the need to reduce the dark current level within the imager. That is, within a charge coupled device (CCD) imager, the dark current shot noise adds in quadrature with the imager readout noise, thereby degrading the useful signal-to-noise ratio. Typically, a thermoelectric cooler is used to reduce the temperature of the CCD imager and, therefore, reduce the imager dark current level. Unfortunately, such a thermoelectric cooler greatly adds to both the weight and power consumption of the resulting imaging device.
Therefore, it is seen to be desirable to provide a night vision imaging apparatus having a dark current that is low enough to avoid the use of a thermoelectric cooler.
The disadvantages of the prior art are overcome by the present invention of an imager architecture in which low imager dark current levels are achieved without auxiliary cooling devices. Imaging apparatus according to an embodiment of the present invention utilizes a line transfer imager with horizontal line readout and vertical complementary metal oxide (CMOS) scanning. Specifically, a two dimensional array of charge coupled devices having relatively small individual areas with respect to the line pitch are read as respective horizontal lines using a low noise CCD output stage. Select lines and a shift register or decoder using CMOS circuits integrated with the CCD lines are used. An optical lens array is used to increase the optical fill factor to compensate for the relatively small area covered by the deletions of the CCD array. Within the CCD array, unused area is filled with drains to reduce dark current and provide blooming control. Moreover, drain and dump gates at the top of the detectors provide additional blooming control and electronic shutter control. In one embodiment of the invention, virtual gate detectors with charge storage are used to simplify the overall device structure and reduce dark current.
A method according to an embodiment of the invention comprises the steps of: clearing dark current from a horizontal charge coupled device (CCD) register to a corresponding horizontal register dump drain; loading a detector signal into the CCD register; selecting the CCD register and reading out the contents of the register; and repeating the preceding steps for each horizontal line of a CCD array.